The present invention relates to a method of manufacturing electrode foil for aluminum electrolytic capacitors and an alternative current (AC) power supply unit used in the manufacturing method. More particularly, the present invention relates to a method of manufacturing and an AC power supply unit involving the technologies most suitably used in an etching of an anode foil for low voltage aluminum electrolytic capacitors.
In recent years as the equipment in which aluminum electrolytic capacitors are used has been reduced in size and enhanced in reliability, requirements from users of the aluminum electrolytic capacitors for a reduction in size, as well as a reduction in cost of the capacitors are rapidly increasing. As a result, greatly increased electric capacity per unit area of electrode foil used in aluminum electrolytic capacitors has been required.
Among the aforementioned electrode foil, an anode electrode foil for low voltage aluminum electrolytic capacitors is produced in the following manner. At first, electrochemical or chemical etching surface treatments are applied to aluminum foil to expand the surface area thereof. Then, the aluminum foil is washed to form a hydrate film on the surface thereof, followed by a drying of the foil to finish the etched foil. Afterwards, an anodizing treatment is applied to the etched foil to perform anode oxidation, thus finishing the production of the anode foil.
As a result of the etching surface treatments applied to aluminum foil, as described above, great number of etched pits are formed step by step inside the aluminum foil as the step of performing the etching process is repeated. The etching processes are performed in a electrolytic solution mainly comprised of hydrochloric acid, while alternating currents are applied.
In general, a mechanism of an alternating current etching process is known as follows. A hydrate film is formed on the surface of aluminum when a cathode current flows on the surface and pits are formed at the places where the hydrate film has defects at the time when an anode current flows. By repeating this reaction with alternating currents being applied, etch pits are formed in the aluminum foil. By devising or improving the composition of the electrolyte and the etching conditions such as a current density and frequencies of the alternating currents, etched foil characteristics such as electric capacity, mechanical strength and the like, have been enhanced.
In the step of performing the etching process, in which alternating currents are applied to aluminum foil in a electrolytic solution that is mainly comprised of hydrochloric acid, a sine wave is used as the waveform of the alternating currents, as shown in FIG. 10, and an increase of the surface area of aluminum foil is obtained by adjusting the frequencies of the sine wave. However, an excessive increase of the surface area of the aluminum foil results in a reduction of mechanical strength of the aluminum foil, thus presenting a problem that there is a limit to the increase in the surface area of aluminum foil.
Although an etching process employing a rectangular wave, a triangular wave and the like, as the waveform of alternating currents have been tried, it became rather difficult to further increase the surface area of aluminum foil. In addition, as is shown in the Japanese Patent Laid-Open Unexamined Publications S51-25439, S61-158133, H2-66925, H2-211614, H10-163073 and the like, a variety of electric current waveforms are proposed for use in the alternating current etching process. However, even with those improved technologies, it is hard to satisfy the requirements for electrode foil for aluminum electrolytic capacitors requested in recent years.
The present invention addresses the problems as described above and aims to provide a method of manufacturing electrode foil for aluminum electrolytic capacitors and an alternating current power supply unit used therein. The method and unit allow the electric capacity of electrode foil to be enhanced via an increase of the surface area of aluminum foil by forming closely formed, high-density etch pits in consideration of such factors as a chemical dissolving reaction, an electrochemical reaction and a diffusion phenomenon involving the aluminum foil.
A method of manufacturing electrode foil for aluminum electrolytic capacitors of the present invention is characterized by applying alternating currents to a pair of electrode plates in a electrolytic solution in etching processing of aluminum foil. The method employs, as the waveforms of alternating currents between the anode and cathode, a deformed sine wave that has a different period of time between a portion where the current rises from zero to a peak value and a portion where the current falls from the peak value to zero. More specifically, during the period of time when the current rises from zero to the peak value, uniform and closely formed etch pits are produced and then, during the next period of time when the current falls from the peak value to zero, etch pits are grown to have a high density, thereby expanding the surface area of aluminum foil efficiently. As a result, the electric capacity of electrode foil for aluminum electrolytic capacitors is increased.
In other words, by applying a great amount of electricity to aluminum foil in such a short period of time when the electric current rises from zero to the peak value, uniform and closely formed etch pits are produced, and then during the next period of time when the electric current falls from the peak value to zero, the etch pits are grown to have a high density while the etch pits being produced, thereby allowing the surface area of aluminum foil to be expanded efficiently.
In addition, when the peak value of the deformed sine wave current is made to be the same as the peak value of a conventional sine wave current, a root-mean-square (rms) value and a mean value of the electric current are made to be the same. This allows the production conditions involving the alternating currents to be easily set in comparison with a case where a rectangular waveform or a triangular waveform is used and consequently, allows the electric capacity per unit amount of electricity to be increased.
A hydrate of aluminum is produced on the cathode of alternating currents and etch pits are formed on the anode from defects of the hydrate of aluminum. The etch pits are uniformly distributed in location without depending on the conditions of the defects of the hydrate, since the current density during the initial period of time when the current rises becomes particularly high. The observation of the current waveform in the anode shows this fact. In addition, it is preferred that the waveforms of the deformed sine wave applied to the anode and cathode are made to be symmetrical.
When the period of time when the current falls exceeds 120 where the period of time when the current rises is taken as 1 (i.e., the period of time when the current rises becomes shorter than another period of time), the reaction during the current rising period becomes too abrupt, thereby producing excessively fine etch pits. These excessively fine etch pits increase the apparent surface area of aluminum foil, but at the same time increase electrical resistance inside the respective etch pits, thereby prohibiting the electric capacity to be utilized effectively. Therefore, an average etch pit diameter ranging from 0.15 xcexcm to 2.8 xcexcm is appropriate. Also, whenever the period of time when the current falls is less than 1.05, etch pits are not allowed to be uniformly distributed in location, thereby preventing the surface area of aluminum foil from being expanded.